The present disclosure relates in general to coatings for protecting metal articles and more particularly to fusion bonded epoxy coatings for pipelines intended for underwater off-shore use, above-ground use or inground implantation.
Various protective pipeline coatings are well known in the art. Additionally, federal regulations require that major pipelines be cathodically protected in order to decrease failures due to corrosion. Cathodic protection is defined as reduction or elimination of corrosion by making the metal a cathode by means of an impressed direct current or attachment to a sacrificial anode, usually magnesium, aluminum, or zinc.
While turning the pipeline structure into a cathode will decrease corrosion in general, breaks and imperfections in the pipeline coating present problems. Initially, these breaks or “holidays” are protected by the negative charge, but with time, and in part due to moisture and minerals in the soil, and in some cases aided by hot pipeline contents, this negative charge may accelerate undercutting and disbondment of the coating system in the areas of holidays.
For these reasons, the pipeline coating art has devoted attention to the task of providing cathodic disbondment resistance to the various protective coating systems employed.
U.S. Pat. No. 3,876,606 (Kehr) relates to thermosetting epoxy resin powder for coating metal articles which are said to exhibit good resistance to cathodic disbondment. The epoxy resin composition includes a homogenous blend of a polyglycidyl ether of a polyhydric phenol having a softening point of 70°-120° C., a specified dihydrazide hardening agent, and at least 15% by volume of barium sulfate and/or calcium carbonate, up to one-third of which may be substituted by mica powder. U.S. Pat. No. 5,108,809 (Patil et al.) describes a coating that includes a synthetic elastomer and/or natural rubber compound, and further includes an amphipathic metal complexing agent as a cathodic disbondment inhibitor.
U.S. Pat. No. 5,859,153 (Kirk et al.) describes a powder coating that includes an epoxy resin and a novolak compound or resin. The novolak compound is said to be useful for enhancing the adhesion of epoxy resin to metal substrates under typical powder coating conditions.
While the foregoing technologies may be useful, different powdered epoxy compositions are needed that not only adhere to metal, but also exhibit enhanced resistance to cathodic disbondment under a variety of moisture and elevated temperature conditions.